1. Field of the Invention
The present invention relates to an engine with a variable combustion chamber which can be modified into a direct-injection-type combustion chamber when the engine operates under low load and a prechamber-type combustion chamber when the engine operates under high load, and more particularly to such an engine which employs alcohol as fuel.
2. Description of the Related Art
Conventional engines use fossil fuel such as gasoline, light oil, or the like, as fuel. However fossil fuel is a limited natural resource. Further, exhaust gases emitted from engines which burn gasoline, light oil, etc. contain harmful substances which are partly responsible for air pollution. Various efforts have been made to eliminate or reduce these shortcomings. These efforts include creation of an engine which uses alcohol as fuel which is chemically or biochemically produced.
Alcohol has a greater latent heat value of vaporization and generates smaller heat than conventional fuels. For example, in order to produce the same amount of heat, it is necessary to supply an engine with 1.6 times as much alcohol as light oil. Since the amount of alcohol injected into an engine cylinder must thereby be larger, air and injected alcohol are not easily mixed together in the combustion chamber, and the time required for the alcohol to be diffused and combusted is longer. Especially when the engine is subjected to high load, the amount of alcohol to be injected is increased and hence the engine output power and the combustion efficiency are lowered.
When 1.6 times as much alcohol as light oil is supplied, then its latent heat of vaporization is about 5.6 times that of light oil. Therefore, alcohol has less of a tendency to be vaporized in the combustion chamber. When the engine is started or the temperature of ambient air is low, alcohol is not fully burned, and unburned gases are emitted from the engine, which results in unpleasant odors and air polluted by harmful substances contained in the emitted unburned gases. An additional problem is that, the injected alcohol may not be ignited, and the engine may not be started.
Various engines with variable compression ratios have been proposed in order to improve starting ability at low temperatures. For example, Japanese Utility Model Publication No. 57(1982)-164224 discloses an engine having a main combustion chamber and an auxiliary chamber communicating therewith. In addition to intake and exhaust valves, a third valve is disposed in the opening of the auxiliary chamber. When the engine is under low load, the opening of the auxiliary chamber is closed by the third valve, giving the engine a higher compression ratio. When the engine is under high load, the third valve is opened on each compression stroke, thus reducing the compression ratio for the prevention of knocking. However, the disclosed engine cannot solve the problems referred to above.